1. Field of the Invention
The present invention relates to a squirrel-cage rotor for an induction motor.
2. Description of the Related Art
A squirrel-cage rotor for an induction motor mainly includes a rotor core, a plurality of bars inserted into slots in the rotor core and end rings joined to the bars at both end faces of the rotor core. In views of manufacturing cost and material cost, the plurality of bars and the end rings are frequently made of aluminum, in one body by die cast molding.
However, in recent years, an induction motor generating high power and low-calorific power has been required. For this purpose, a conductor part of the rotor of the induction motor may be made of metal of which resistance is lower than that of aluminum, for example, copper or copper alloy.
Additionally, it is required to speed up the operation of a motor. Nevertheless, since copper or copper alloy has a melting point higher than the melting point of aluminum, there is a possibility that the formation of a rotor's conductor part by die casting exerts an influence on magnetic steel sheets forming the rotor core. For this reason, it is difficult to adopt die-casting in forming the rotor's conductor part from metal, such as copper or copper alloy.
Therefore, when forming the rotor's conductor part using metal, such as copper or copper alloy, it is performed to manufacture a plurality of bars and end rings having a plurality of holes formed to receive these bars and continuously join the bars and the end rings by brazing etc. However, to produce an increase in temperature up to about 800° C. at brazing causes the metal to be annealed to lower its strength. Thus, there is a possibility that if rotating a motor containing the so-produced rotor at high speed, the holes of the end rings are deformed due to centrifugal force, causing the rotor to be damaged.
In prior art, therefore, attaching reinforcing members to the outside of the end rings is well known. For instance, Japanese Patent No. 2911315 discloses a technique of attaching a reinforcing member having a U-shaped cross section to an end face of the end ring. Additionally, there is proposed another technique of securing a reinforcing member (end ring holder) having a L-shaped cross section or a reinforcing ring to the end face of the end ring by welding, brazing or cold/hot isotropic pressing, etc., in Japanese Unexamined Patent Application Publication Nos. 10-127022 and 3-261354 and also Japanese Patent No. 2838896.
Additionally, Japanese Unexamined Patent Application Publication No. 2-119557 proposes a method of attaching a holding ring made from iron-based high strength material to the outside of the end ring and successively joining the former to the latter by arc welding or electron beam welding, etc., Further, Japanese Unexamined Patent Application Publication No. 2011-10498 discloses a technique of interposing an end plate, which is provided with slots each having an area larger than that of a magnetic steel sheet, between the end ring and the rotor core, so that a concentration of stress is alleviated to improve the reliability.
However, as the technique disclosed in Japanese Patent No. 2911315 assumes that the bars and the end rings are integrally formed in one body by “aluminum” die-casting, the technique of Japanese patent No. 2911315 cannot be employed in case of forming the bars and the end ring independently of each other. Japanese Unexamined Patent Application Publication Nos. 10-127022 and 3-261354 and also Japanese Patent No. 2838896 have a common problem that the junction condition between the bars and the end ring is cannot be easily seen from the outside.
Additionally, the method disclosed in Japanese Unexamined Patent Application Publication No. 2-119557 produces relatively-numerous junction points between the holding ring and the an end face of the end ring by arc welding or electron beam welding and furthermore, cans have to be removed finally. Therefore, the method in Japanese Unexamined Patent Application Publication No. 2-119557 accompanies an increased number of manufacturing steps to cause a manufacturing cost to increase. Further, if the area of each slot gets larger in Japanese Unexamined Patent Application Publication No. 2011-10498, then the reliability of the resultant rotor of Japanese Unexamined Patent Application Publication No. 2011-10498 is not always improved since a wall thickness between the slots and the outer diameter of the rotor is reduced.
Under such a situation, an object of the present invention is to provide a squirrel-cage rotor for an induction motor, which can be manufactured by any manufacturing method but die-casting and which allows end rings to be reinforced with use of easily-available material and also allows the junction condition between the bars and the end rings to be confirmed from the outside easily.